Here is some good traditional theology on the nature of faith. The concept of faith these days is not often very clearly defined or explained. It is often described in such a way that gives the impression of it being merely a thing of sentiment - which essentially boils down to the Modernist understanding of faith; whereas the traditional understanding of faith is that it is a supernatural virtue of the intellect. Garrigou-Lagrange here discusses the nature of faith with a theological precision worthy of St. Thomas himself. This passage is taken from his book, Reality - A Thomistic Synthesis.
The theological virtues and their acts, like faculties, virtues, and acts in general, are specifically proportioned to their formal object. The profound import of this principle went unrecognized by Scotus and by the Nominalists and their successors, as is clear from the controversies which, from the fourteenth century onwards, have never ceased.
Faith, says St. Thomas,  has as its material object all truths revealed by God, but chiefly the supernatural mysteries not accessible to any natural intelligence human or angelic. But the formal object of faith, its formal motive of adherence, is God's veracity,  which presupposes God's infallibility.  The veracity here in question is that of God as author, not merely of nature, but of grace and glory, since the revealed mysteries, the Trinity, for example, and the redemptive Incarnation, are essentially supernatural. Let us quote the saint's own words:
"Faith, considered in its formal object, is nothing else than God, the first truth. For faith assents to no truth except in so far as that truth is revealed. Hence the medium by which faith believes is divine truth itself.  Again: "The formal object of faith is the first truth, adherence to which is man's reason for assenting to any particular truth."  Once more: "In faith we must distinguish the formal element, i. e.: the first truth, far surpassing all the natural knowledge of any creature; and second, the material element, i. e.: the particular truth, to which we adhere only because we adhere to the first truth."  Lastly: "The first truth, as not seen but believed, is the object of faith, by which object we assent to truths only as proposed by that first truth." .
Thomists, explaining these words, note that the formal object of any theological virtue must be something uncreated, must be God Himself. Neither the infallible pronouncements of the Church nor the miracles which confirm those pronouncements are the formal object of faith, though they are indispensable conditions. Faith, therefore, being specifically proportioned to a formal object which is essentially supernatural, must itself be essentially supernatural. Again we listen to Thomas.
"Since the act by which man assents to the truths of faith is an act beyond man's nature, he must have within, from God, the supernatural mover, a principle by which he elicits that act."  And again: "The believer holds the articles of faith by his adherence to the first truth, for which act he is made capable by the virtue of faith." .
In other words the believer, by the infused virtue of faith and by actual grace, adheres supernaturally to the formal motive of this theological virtue, in an order which transcends all apologetic arguments, based on evident miracles and other signs of revelation. His act of adherence is not discursive, but simple, since all through it is one and the same act. That act can be expressed in three ways:  I believe God who reveals,  I believe what has been revealed concerning God,  I believe unto God.  But by these three expressions, says St. Thomas,  we designate, not different acts of faith, but one and the same act in different relations to one and the same object, as, we may add in illustration, the eye, by one and the same act of vision, sees both light and color.
Faith, therefore, has a certitude essentially supernatural, surpassing even the most evident natural certitude, whether that of wisdom, of science, or of first principles.  God's authority claims our infallible adherence in an order far higher than apologetic reasoning, which is prerequired for credibility, i. e.: that the mysteries proposed by the Church are guaranteed by signs manifestly divine, and are therefore evidently credible. Even for the willingness to believe,  actual grace is prerequired.
This essential supernaturalness of faith is not admitted by Scotus, nor the Nominalists, nor their successors. Scotus says that the distinction of grace from nature is not necessary, but contingent, dependent on the free choice of God, who might have given us the light of glory as a characteristic of our nature,  since a natural act and a supernatural act can each have the same formal object.  Neither is infused faith necessary by reason of a supernatural object, because the formal object of theological faith is not higher than acquired faith.  Lastly, the certitude of infused faith is based on acquired faith in the veracity of the Church, which veracity is itself founded on miracles or other signs of revelation. Otherwise, so he claims, we would regress to infinity. This same doctrine is upheld by the Nominalists.  Thence it passes to Molina,  to Ripalda,  and with slight modification to de Lugo  and to Franzelin.  Vacant  shows clearly wherein this theory differs from Thomistic teaching.
Thomists reply as follows: The formal motive of infused faith is the veracity of God, the author of grace, and this motive, inaccessible to any natural knowledge whatsoever, must be attained by an infused virtue. If acquired faith, which even demons have, were sufficient, then infused faith would not be absolutely necessary, but would be, as the Pelagians said, a means for believing more easily. Against the Pelagians the Second Council of Orange defined the statement that grace is necessary even for the beginning of faith, for the pious willingness to believe.
Resting on the principle that habits are specifically differentiated by their formal objects, Thomists, since the days of Capreolus, have never ceased to defend the essential supernaturalness of faith, and its superiority to all natural certitude. On this point Suarez  is in accord with Thomists, but with one exception. To believe God who reveals, and to believe the truths revealed concerning God, are for him two distinct acts, whereas for Thomists they are but one.
Thomists are one in recognizing that the act of infused faith is founded  on the authority of God who reveals, and hence that God is both that by which and that which we believe,  as light, to illustrate, is both that by which we see, and that which is seen, when we see colors.  But this authority of God can be formal motive only so far as it is infallibly known by infused faith itself. Were this motive known only naturally, it could not found a certitude essentially supernatural.
We may follow this doctrine down a long line of Thomists. Capreolus  writes: "With one and the same act I assent, both that God is triune and one, and that God revealed both truths. By one and the same act I believe that God cannot lie,  and that what God says of Himself is true."  Cajetan  writes: "Divine revelation is both that by which (quo) and that which (quod) I believe. Just as unity is of itself one without further appeal, so divine revelation, by which all else is revealed, is accepted for its own sake and not by a second revelation. One and the same act accepts the truth spoken about God and the truthfulness of God who speaks."  "This acceptance of the first truth as revealing, and not that acquired faith by which I believe John the Apostle, or Paul the Apostle, or the one Church, is the ultimate court of appeal. The infused habit of faith makes us adhere to God as the reason for believing each and every revealed truth. 'He that believeth in the Son of God hath the testimony of God in himself.'"  This same truth you will find in Sylvester de Ferraris,  in John of St. Thomas,  in Gonet,  in the Salmanticenses,  and in Billuart. .
All Thomists, as is clear from these testimonies, rest on the principle so often invoked by St. Thomas: Habits and acts, since they are specifically differentiated by their formal objects, are in the same order as are those objects. This principle is the highest expression of the traditional doctrine on the essential supernaturalness of faith, and of faith's consequent superiority over all natural certitude. Let us repeat the doctrine in a formal syllogism, whereof both major and minor are admitted by all theologians.
We believe infallibly all that is revealed by God, because of the authority of divine revelation, and according to the infallible pronouncements of the Church. But revelation and the Church affirm, not only that the revealed mysteries are truths, but also that it is God Himself who has revealed those mysteries. Hence we must believe infallibly that it is God Himself who has revealed these mysteries.
Note, as corollary, that the least doubt on the existence of revelation would entail doubt on the truth of the mysteries themselves. Note further that infallible faith in a mystery as revealed presupposes, by the very fact of its existence,  that we believe infallibly in the existence of divine revelation, even though we do not explicitly reflect on that fact. .
An objection arises. St. Thomas teaches that one and the same truth cannot be simultaneously both known and believed. But, by the miracles which confirm revelation, we know the fact of revelation. Hence we cannot simultaneously believe them supernaturally. In answer, Thomists point out that revelation is indeed known naturally as miraculous intervention of the God of nature, and hence is supernatural in the mode of its production, like the miracle which confirms it. But revelation, since it is supernatural in its essence, and not merely in the mode of its production, can never be naturally known, but must be accepted by supernatural faith. By one and the same act, to repeat St. Thomas,  we believe the God who reveals and the truth which He reveals.
"Faith," says the Vatican Council,  "is a supernatural virtue by which we believe that all that God reveals is true, not because we see its truth by reason, but because of the authority of God who reveals." By the authority of God, as the phrase is here used, we are to understand, so Thomists maintain, the authority of God, not merely as author of nature and of miracles, which are naturally known, but the authority of God as author of grace, since revelation deals principally with mysteries that are essentially supernatural.
Is this distinction, between God the author of nature and God the author of grace, an artificial distinction? By no means. It runs through all theology, particularly the treatise on grace. Without grace, without infused faith, we cannot adhere to the formal motive of faith, a motive far higher than the evidence of credibility furnished by miracles. The believer holds the articles of faith, says St. Thomas,  simply because he believes and clings to the first truth, which act is made possible by the habit of faith. Thus the believer's act, essentially supernatural and infallible, rises immeasurably above acquired faith as found in the demon, whose faith is founded on the evidence of miracles, or in the heretic who holds certain dogmas, not on the authority of God which he has rejected, but on his own judgment and will.
The consequences of this doctrine for the spiritual life are very pronounced. We see them in the teaching of St. John of the Cross on passive purification of the spirit. Faith is purged of all human alloy in proportion to its unmixed adherence to its formal motive, at a height far above the motives of credibility, including all accessory motives, life in a believing community, say, which facilitates the act of faith. .
The gifts which correspond to the virtue of faith are, first, understanding, which enables us to penetrate the revealed mysteries,  second, knowledge, which illumines our mind on the deficiency of second causes, on the gravity of mortal sin, on the emptiness of a worldly life, on the inefficacy of human concurrence in attaining a supernatural end.  This gift thus also facilitates a life of hope for divine gifts and eternal life.
1141 Against Gottschalk. Cf. PL, CXXVI, 123.
1142 See our work, La predestination des saints et la grace 1936, pp. 257-64, 341-45, 141-69. Cf. "Le fondement supreme de la distinction des deux graces suffisante et efiicace" in Rev. thom.: May-June, 1937; "Le dilemme: Dieu determinant ou determine," Ibid.: 1928, pp. 193-210.
1143 I Cor. 4: 7.
1194 IIa IIae, q. 1, a. 1.
1195 Ibid.: q. 2, a. 2.
1196 Ibid.: q. 5, a. 1.1197 Ibid.: q. 4, a. 1.
1198 Ibid.: q. 6, a. 1.1199 Ibid.: q. 5, a. 3, ad 1.1200 Ibid.: q. 2, a. 2, ad 3.1201 Credo Deo revelanti.
1202 Credo Deum revelatum.1203 Credo in Deum.1204 IIa IIae, q. 2, a. 2, ad 3.1205 Ibid.: q. 4, a. 8.1206 Pius credulitatis affectus.1207 In I Sent.: dist. III, q. 3, nos. 24f.1208 In III Sent.: dist. XXXI, no. 4.1209 Ibid.: dist. XXIII, q. 1, a. 8.
1210 Biel, In III Sent.: dist. XXIII, q. 2.
1211 Concordia, q. 14, a. 13, disp. XXXVIII, Paris, 1876, p. 213.
1212 De ente supernat.: Bk. III, dist. XLIV, no. 2; dist. XLV, no. 37.
1213 De fide, disp. IX, sect I, nos. 2, 3; disp. 1, sect. I, nos. 77, 100, 104.
1214 De divina traditione, pp. 692, 616.
1215 Etudes sur le concile du Vatican, II, 75 ff.
1216 De gratia, Bk. II, chap. 11; De fide, Part 1, disp. III, sect. 6, 8, 12.
1217 Ultimo resolvitur.
1218 Id quo et quod creditur.
1219 Id quo et quod videtur simul cum coloribus.
1220 In III Sent.: d. 24, q. 1, a. 3.
1221 Credo Deo.1222 Credo Deum.1223 In lllam lIIae, q. 1, a. 1, no. 11.
1224 See Ibid.: q. 2, a. 2.
1225 I John 5:10.1226 In Cont. Gent.: I, 6; III, 40, § 3.
1227 De gratia, disp. XX, a. 1, nos. 7, 9; De fide q. 1, disp. 1, a. 2, nos. 1, 4.
1228 De gratia, disp. 1, a. 2, § 1, nos. 78, 79, 93; De fide, disp. 1, a. 2, no. 55.1229 De gratia, disp. III, dub. 3, nos. 28, 37, 40, 45, 48, 49, 52, 58, 60, 61; De fide, disp. 1, dub. 5, nos. 163, 169.
1230 De gratia, diss. III, a. 2, § 2; De fide, diss. 1, a. l, obj. 3, inst. 1. See also Gardeil, La credibilite et l'apologetique, 2nd ed.: Paris, 1912, pp. 61, 92, 96, and in Dict. de theol. cath.: s. v. Credibilite. See also Scheeben, Dogmatik, 1, § 40, nos. 681, 689; § 44, nos. 779805. And for extended treatment, see our work, De revelatione, Rome, 3rd ed.: 1935, I, 458-511.
1231 In actu exercito.
1232 In actu signato.
1233 IIa IIae, q. 2, a 2, ad 3.
1234 Sess. III, chap. 3.
1235 Cf. IIa IIae, q. s, a. 3, ad 1. See also John of St. Thomas, De gratia, disp. XX, a. 1, nos. 7-9; De fide, q. 1, disp. 1, a2, nos. 1-8; also the Salrnanticenses, De gratia disp. III, dub. 3, nos. 28-37, 40-49, 52-61.
1236 For more extended treatment, see our work, L'amour de dieu et la croix de Paris, 2nd ed.: 1939, 11, 575-97.
1237 IIa IIae, q. 8.
1238 Ibid.: q. 9.1239 Ibid.: q. 17-22.1240 Ibid.: q. 17, a. 1, 2, 4, 5. Deus auxilians.